Multiple therapy cardiac assist device having battery voltage safety monitor

ABSTRACT

A cardiac assist system which provides simultaneous cardiac assistance during a confirmed cardiac arrythmia, but which provides for the inhibition of the cardiac assistance in the event battery voltage drops below a predefined point. In particular the present invention features a battery voltage safety limit during the deliver VF therapy state. During VF therapy state the system provides skeletal muscle stimulation while charging a defibrillation output capacitor. The battery voltage safety limit is set above a threshold which will insure proper device functionality, i.e. above the power-on-reset threshold (hereafter &#34;POR&#34;). If the battery voltage safety limit is reached the system inhibits the skeletal muscle stimulation to thereby avoid reaching the POR threshold, but yet still maintain the charging of the defibrillation output capacitor. Once charging of the defibrillation output capacitor is completed a synchronization routine is begun and defibrillation therapy is delivered. In such a manner the present invention prevents the dual demands of skeletal muscle stimulation along with charging of the defibrillation output capacitor to cause the battery voltage to collapse and drop below the power-on-reset threshold.

FIELD OF THE INVENTION

The present invention generally relates to cardiac assist systems, including cardiomyoplasty for the treatment of patients needing augmented cardiac output. More specifically, the present invention relates to a cardiac assist system which provides simultaneous cardiac assistance during a confirmed cardiac arrythmia, but which provides for the inhibition of the cardiac assistance in the event battery voltage drops below a predefined point.

BACKGROUND OF THE INVENTION

Cardiac assist systems aid patients with chronically and unacceptably low cardiac output who cannot have their cardiac output raised to acceptable levels by traditional treatments, such as drug therapy. One particular type of cardiac assist system currently used is a cardiomyoplasty.

Essentially a cardiomyoplasty provides a muscle-powered cardiac assist system. As seen in U.S. Pat. No. 4,813,952 of Khalafalla, incorporated herein by reference, the cardiomyoplasty is a cardiac assist system powered by a surgically-modified muscle tissue, such as the latissimus dorsi. In particular, the latissimus dorsi is wrapped around the heart. An implantable pulse generator is provided. The implantable pulse generator senses contractions of the heart via one or more sensing leads and stimulates the appropriate nerves of the muscle tissue with burst signals to cause the muscle tissue to contract in synchrony with the heart. As a result, the heart is assisted in its contractions, thereby raising the stroke volume and thus cardiac output. Besides delivering therapeutic electrical pulses to the muscle, the pulse generator is quite often also coupled so as to also provide therapeutic electrical pulses to the heart. See, for example, U.S. Pat. No. 4,735,205 of Chachques et al., incorporated herein by reference.

Patients with chronic cardiac output deficiencies, although treatable through cardiomyoplasty, face an increased risk for cardiac arrhythmic episodes, such as ventricular tachycardia or fibrillation. These arrhythmic episodes may be life-threatening.

In order to treat these potentially life-threatening cardiac arrhythmias, some cardiac assist systems have been proposed which combine both a muscle stimulator as well as a cardiac pacer-cardioverter-defibrillator. In such a manner a patient who has had a cardiomyoplasty may, in addition to receiving muscle-powered cardiac assistance, also receive various types of therapeutic cardiac electrical stimulation. One example of such a system may be seen in the U.S. Pat. No. 5,251,621 issued to Collins and entitled "Arrhythmia Control Pacer Using Skeletal Muscle Cardiac Graft Stimulation." In particular the device of Collins provides muscle stimulation while the arrythmia is being confirmed and also immediately prior to the delivery of a defibrillation shock.

One problem associated with devices which combine both a muscle stimulator as well as a cardiac pacer-cardioverter-defibrillator, however, is the extra power requirements of simultaneously delivering skeletal muscle stimulation along with charging a defibrillation output capacitor.

In a new device, this extra power requirement only causes a slightly longer period required for the charging of the defibrillation output capacitor. In general this is an acceptable side effect since the benefits of cardiac augmentation during fibrillation outweigh the increase time required for the delivery of therapy. In particular, the increased cardiac perfusion from the skeletal muscle stimulation lowers the defibrillation threshold.

In devices having less than a full battery, however, the competing demands of concurrent skeletal muscle stimulation along with charging of the defibrillation output capacitor may have more serious consequences. In particular, the battery voltage could collapse and drop below the power-on-reset (hereafter "POR") threshold. If that were to occur the device would reset itself. Functionality would be temporarily lost and all ongoing therapies interrupted or aborted. The result would be any defibrillation therapy would not be delivered until a significant amount of time had elapsed.

Previously, others have attempted to provide some means of protecting against excessively low voltages within a device. U.S. Pat. No. 4,868,908 to Pless entitled "Power Supply Down-Conversion, Regulation and Low Battery Detection System" discloses a low battery detect circuit which shuts down high current circuitry in the system in the event that battery voltage drops to a level such that the regulated output voltage is endangered. Of course, in devices featuring concurrent skeletal muscle stimulation along with charging of the higher voltage, higher current defibrillation output capacitor, shutting down the higher voltage, higher current circuitry is precisely the worst possible thing to do. During VF, the first priority is the delivery of a defibrillation shock, as such the charging of the higher voltage, higher current defibrillation output capacitor should take precedence over the lower voltage, lower current muscle stimulation therapies.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a system which insures proper functionality of the device is maintained at all times.

It is a further object of the present invention to provide a system which shuts down low current, low voltage circuitry in the event the battery voltage could collapse and drop below the power-on-reset (hereafter "POR") threshold while also maintaining the charging of the high current, high voltage circuitry.

These and other objects are met by the present invention which features a battery voltage safety limit during the deliver VF therapy state. During VF therapy state the device provides skeletal muscle stimulation while charging a defibrillation output capacitor. The battery voltage safety limit is set above the POR threshold. If the battery voltage safety limit is reached the device inhibits the skeletal muscle stimulation to thereby avoid reaching the POR threshold, but yet still maintain the charging of the defibrillation output capacitor. Once charging of the defibrillation output capacitor is completed a synchronization routine is begun and defibrillation therapy is delivered. In such a manner the present invention prevents the dual demands of skeletal muscle stimulation along with charging of the defibrillation output capacitor to cause the battery voltage to collapse and drop below the power-on-reset threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the present invention will be best appreciated with reference to the detailed description of the invention in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates an example of a system for performing both long-term stimulation of skeletal muscles for cardiac assistance using systolic augmentation as well as direct electrical stimulation of a heart according to the present invention.

FIG. 2 is a functional schematic diagram of an implantable pulse generator used in the system of the present invention.

FIG. 3 is an illustration of detection interval ranges employed in a preferred embodiment of the present invention.

FIG. 4 is an arrythmia detection/therapy muscle state diagram of the present invention.

FIG. 5 is a timing diagram showing the relationship between muscle stimulation, cardiac events, and a defibrillation charge cycle.

FIG. 6 is a timing diagram showing the relationship between muscle stimulation and cardiac events of an alternate embodiment.

FIG. 7 depicts an alternate muscle stimulation burst which may be used with the present system.

FIG. 8 depicts an alternate embodiment of the muscle catch stimulation which may be used with the present system.

FIG. 9 depicts an alternate embodiment of the muscle catch stimulation which may be used with the present system.

FIG. 10 is a timing diagram showing the relationship between muscle stimulation, cardiac events, device states, a defibrillation charge cycle, and battery voltage and the effect simultaneous muscle stimulation may have on the overall battery voltage and time to reach defibrillation charge.

FIG. 11 is a timing diagram showing the relationship between muscle stimulation, cardiac events, a defibrillation charge cycle, and battery voltage and the effect of the battery voltage safety limit of the present invention.

FIG. 12 is a flowchart showing the operation of the system and in particular the operation of the battery voltage safety limit feature of the present invention.

FIG. 13 is a flowchart showing the operation of an alternate embodiment of the battery voltage safety limit feature of the present invention.

The drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention employs a sensor to monitor cardiac electrical activity and cardiac demand in a skeletal muscle-powered cardiac assist system (hereinafter referred to as "CAS"). A basic CAS may be configured in a variety of ways as described in the aforementioned patent to Khalafalla. One specific configuration is discussed herein simply as an illustration. The present invention, however, may be used in any system concerning cardiac augmentation using skeletal muscle, such as aortic counterpulsation or a skeletal muscle ventricle. Thus it should be understood the particular configuration illustrated is not intended to limit the present invention.

The System of the Present Invention

FIG. 1 illustrates an example of a system 1 for performing long-term stimulation of skeletal muscles for cardiac assistance using systolic augmentation as well as direct electrical stimulation of a heart 2. As seen, skeletal muscle graft 3 is positioned about the heart 2. In the preferred embodiment the latissimus dorsi muscle is used for the skeletal muscle graft, as is well known in the art. The longitudinal fibers of the muscle graft 3 are oriented generally perpendicular to the longitudinal axes of the right ventricle 4, left ventricle 5 and interventricular septum 10 of the heart. Muscle graft 3 is positioned in this manner so that when it is stimulated, muscle graft 3 compresses ventricles 4, 5 and particularly left ventricle 5, to thereby improve the force of right and left ventricular contraction. In such a manner the overall hemodynamic output of heart 2 is increased.

In a preferred configuration, muscle graft 3 is wrapped around the heart 2 and fixedly attached to itself to form a cup-shaped "sling," using running sutures 12. Alternatively, muscle graft 3 may be attached to heart 2 using running sutures 13 as illustrated.

As seen, electrical stimulation and sensing of heart 2 is accomplished through lead 15. In particular, lead 15 electrically couples pulse generator 6 to heart 2. Lead 15 provides cardiac pacing as well as defibrillation therapies. In the preferred embodiment lead 15 is the model 6936 tri-polar TRANSVERSE lead from Medtronic Inc., Minneapolis, Minn. As seen, lead 15 is implanted in right ventricle 4 such that bi-polar pacing electrode assembly 16 is in the right ventricular apex and defibrillation coil 17 is within the right ventricle 4. Although in the preferred embodiment a single lead is provided for pacing as well as defibrillation therapies, other types of lead configurations, such as multiple transvenous or subcutaneous or any combination thereof, may be used.

Muscle graft 3 is electrically stimulated through a pair of leads 21, 22. In particular leads 21, 22 couple pulse generator 6 to skeletal muscle graft 3. In the preferred embodiment leads 21, 22 are the model 4750 intramuscular lead from Medtronic, Inc., Minneapolis, Minn. As seen, each lead 21, 22 extends from pulse generator 6 to latissimus dorsi muscle graft 3. The electrodes (not shown) of each lead 21, 22 are placed to cause muscle graft 3 to contract when electrically stimulated, as is well known in the art. Other types of leads or electrodes, however, may be used, such as epimysial or neuromuscular leads or nerve cuff electrodes.

The pulse Generator of the Present Invention

FIG. 2 is a functional block diagram of a pulse generator 6 in which the present invention may usefully be practiced. This diagram should only be taken, however, as exemplary of the type of device in which the invention may be embodied and not as limiting. It is believed the invention may usefully be practiced in a wide variety of device implementations. For example, the invention is also believed practicable in conjunction with the implantable muscle stimulator-pacemaker-cardioverters-defibrillators disclosed in U.S. Pat. No. 5,251,621 issued to Collins entitled "Arrhythmia Control Pacer Using Skeletal Muscle Cardiac Graft Stimulation."

The device is illustrated as being provided with six electrodes, 500, 502, 504, 506, 508, 572 and 574. Electrodes 500 and 502 may be a pair of electrodes located in the ventricle and mounted to a lead 15 as discussed above. Electrode 504 may correspond to a remote, indifferent electrode located on the housing of pulse generator 6. Electrodes 506 and 508 may correspond to large surface area defibrillation electrodes located within the right ventricle, coronary sinus, superior vena cava or may also be located subcutaneous, located on or part of the device housing or to the epicardium. Electrodes 572 and 574 are muscle stimulation electrodes coupled to the skeletal muscle wrap 3, as discussed above.

Electrodes 500 and 502 are switchable through switch matrix 512 to the R-wave detector circuit, comprising band-pass filter circuit 514, auto threshold circuit 516 for providing an adjustable sensing threshold as a function of the measured R-wave amplitude and comparator 518. A signal is generated on R-out line 564 whenever the signal sensed between electrodes 500 and 502 exceeds the present sensing threshold defined by the auto threshold circuit 516. As illustrated, the gain on the band pass amplifier 514 is also adjustable by means of a signal from the pacer timing and control circuitry 520 on GAIN ADJ line 566.

The operation of this R-wave detection circuitry may correspond to that disclosed in commonly assigned U.S. Pat. No. 5,118,824, issued to Keimel and incorporated herein by reference. However, alternative R-wave detection circuitry such as that illustrated in U.S. Pat. No. 4,819,643, issued to Menken and U.S. Pat. No. 4,880,004, issued to Baker et al., both incorporated herein by reference, may also be employed.

The threshold adjustment circuit 516 sets a threshold corresponding to a predetermined percentage of the amplitude of a sensed R-wave, which threshold decays to a minimum threshold level over a period of less than three seconds thereafter, similar to the automatic sensing threshold circuitry illustrated in the article "Reliable R-Wave Detection from Ambulatory Subjects", by Thakor et al., published in Biomedical Science Instrumentation, Vol. 4, pp. 67-72, 1978.

It is preferable that the threshold level not be adjusted in response to paced R-waves, but instead should continue to approach the minimum threshold level following paced R-waves to enhance sensing of low level spontaneous R-waves associated with tachyarrhythmias. The time constant of the threshold circuit is also preferably sufficiently short so that minimum sensing threshold may be reached within 1-3 seconds following adjustment of the sensing threshold equal to 70-80% of the amplitude of a detected spontaneous R-wave. The invention may also be practiced in conjunction with more traditional R-wave sensors of the type comprising a band pass amplifier and a comparator circuit to determine when the band-passed signal exceeds a predetermined, fixed sensing threshold.

Switch matrix 512 is used to select which of the available electrodes are coupled to band pass amplifier 534. Under control of microprocessor 524, switch matrix directs delivery of electrical stimulation pulses to cardiac tissue and the skeletal muscle wrap. Selection of the switch matrix settings is controlled by the microprocessor 524 via data/address bus 540. Signals from the selected electrodes are passed through band-pass amplifier 534 and into multiplexer 532, where they are convened to multi-bit digital signals by A/D converter 530, for storage in random access memory 526 under control of direct memory address circuit 528. Multiplexer 532 further receives voltage from battery 537 via VBATT 536.

Amplifier 534 may be a broad band pass amplifier, having a band pass extending for approximately 0.5 to 200 hertz. The filtered EGM signals from amplifier 534 are passed through multiplexer 532, and digitized in A-D converter circuitry 530. The digitized data may be stored in random access memory 526 under control of direct memory address circuitry 528.

The occurrence of an R-wave detect signal on line 564 is communicated to microprocessor 524 via data/address bus 540, and microprocessor 524 notes the time of its occurrence.

The remainder of the circuitry is dedicated to the provision of muscle stimulation, cardiac pacing, cardioversion and defibrillation therapies. The pacer timing/control circuitry 520 includes programmable digital counters which control the basic time intervals associated with cardiac pacing and muscle stimulation. The durations of these intervals are determined by microprocessor 524, and are communicated to the pacing circuitry 520 via address/data bus 540. Pacer timing/control circuitry also determines the amplitude of the muscle stimulation and cardiac pacing pulses and the gain of band-pass amplifier, under control of microprocessor 524.

During cardiac pacing or muscle stimulation, the escape interval counter within pacer timing/control circuitry 520 is reset upon sensing of an R-wave as indicated by a signal on line 564, and on timeout triggers generation of a pacing pulse by pacer output circuitry 522, which is coupled to electrodes 500 and 502 or electrodes 572 and 574. The escape interval counter is also reset on generation of a cardiac pacing pulse, and thereby controls the basic timing of cardiac pacing functions, including anti-tachycardia pacing and subsequent muscle stimulation. The duration of the interval deemed by the escape interval timer is determined by microprocessor 524, via data/address bus 540. The value of the count present in the escape interval counter when reset by sensed R-waves may be used to measure the duration of R-R intervals, to detect the presence of tachycardia and change muscle stimulation parameters.

Microprocessor 524 operates as an interrupt driven device, and responds to interrupts from pacer timing/control circuitry 520 corresponding to the occurrence of sensed R-waves and corresponding to the generation of cardiac pacing and muscle stimulation pulses. These interrupts are provided via data/address bus 540. Any necessary mathematical calculations to be performed by microprocessor 524 and any updating of the values or intervals controlled by pacer timing/control circuitry 520 and switch matrix 512 take place following such interrupts.

In the event that a tachyarrhythmia is detected, and an antitachyarrhythmia pacing regimen is desired, appropriate timing intervals for controlling generation of anti-tachycardia pacing therapies are loaded from microprocessor 524 into the pacer timing/control circuitry 520 and switch matrix 512.

Similarly, in the event that generation of a cardioversion or defibrillation pulse is required, microprocessor 524 employs the counters in timing and control circuitry 520 to control timing of such cardioversion and defibrillation pulses, as well as timing of associated refractory periods during which sensed R-waves are ineffective to reset the timing circuitry.

Further, in the event the onset of a tachyarrhythmia is detected, but not yet confirmed, the filtered and digitized EGM available at A/D 530 will be compared by microprocessor 524 with a value from RAM 526. Measured values above set will continue detection. Values below set confirm the arrhythmia if more than 50% of the X out of Y have been detected. In the preferred embodiment X and Y are programmable counts corresponding to the VFNID and the fibrillation event buffer memory (located in the RAM 526) respectively, both of which are discussed in more detail below with regards to the VF counting mode state 34 seen in FIG. 4. Microprocessor 524 will then initiate a therapy if programmed to do so.

In response to the detection of fibrillation or a tachycardia requiring a cardioversion pulse, microprocessor 524 activates cardioversion/defibrillation control circuitry 554, which initiates charging of the high voltage capacitors 556, 558, 560 and 562 via charging circuit 550, under control of high voltage charging line 552. During charging, microprocessor 524 enables pacer/timing control 520 to pace out 522 and switch matrix 512 to deliver muscle stimulation pulses until the high voltage capacitors 556 are sufficiently charged. The voltage on the high voltage capacitors is monitored via VCAP line 538, which is passed through multiplexer 532, and, in response to reaching a predetermined value set by microprocessor 524, results in generation of a logic signal on CAP FULL line 542, terminating charging. The CAP FULL line 542 signal is sent over DATA/ADDRESS 540 to the pace timer/control 520, which then inhibits delivery of the muscle stimulation pulses.

Thereafter, delivery of the timing of the defibrillation or cardioversion pulse is controlled by pacer timing/control circuitry 520. One embodiment of an appropriate system for delivery and synchronization of cardioversion and defibrillation pulses, and controlling the timing functions related to them is disclosed in more detail in the commonly assigned U.S. Pat. No. 5,188,105 by Keimel, Method and Apparatus for Detecting and Treating a Tachyarrhythmia, incorporated herein by reference. Any known cardioversion or defibrillation pulse generation circuitry, however, is believed usable in conjunction with the present invention. For example, circuitry controlling the timing and generation of cardioversion and defibrillation pulses as disclosed in U.S. Pat. No. 4,384,585, issued to Zipes, in U.S. Pat. No. 4,949,719 issued to Pless et al., cited above, and in U.S. Pat. No. 4,375,817, issued to Engle et al., all incorporated herein by reference may also be employed. Similarly, known circuitry for controlling the timing and generation of anti-tachycardia pacing pulses as described in U.S. Pat. No. 4,577,633, issued to Berkovits et al., U.S. Pat. No. 4,880,005, issued to Pless et al., U.S. Pat. No. 7,726,380, issued to Vollmann et al. and U.S. Pat. No. 4,587,970, issued to Holley et al., all of which are incorporated herein by reference may also be used.

In modern cardiac pulse generators, the particular anti-tachycardia and defibrillation therapies are programmed into the device ahead of time by the physician, and a menu of therapies is typically provided. For example, on initial detection of tachycardia, an anti-tachycardia pacing therapy may be selected. On re-detection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher level cardioversion pulse therapy may be selected thereafter. Prior art patents illustrating such pre-set therapy menus of antitachyarrhythmia therapies include the above-cited U.S. Pat. No. 4,830,006, issued to Haluska, et al., U.S. Pat. No. 4,727,380, issued to Vollmann et al. and U.S. Pat. No. 4,587,970, issued to Holley et al. The present invention is believed practicable in conjunction with any of the known anti-tachycardia pacing and cardioversion therapies, and it is believed most likely that the invention of the present application will be practiced in conjunction with a device in which the choice and order of delivered therapies is programmable by the physician, as in current cardiac pulse generators.

In addition to varying the therapy delivered following a failed attempt to terminate a tachyarrhythmia, it is also known that adjustment of detection criteria may be appropriate. For example, adjustment may comprise reducing the number of intervals required to detect a tachyarrhythmia to allow a more rapid re-detection or by changing the interval ranges to bias detection towards detection of ventricular fibrillation, for example as disclosed in U.S. Pat. No. 4,971,058, issued to Pless et al and incorporated herein by reference.

In the present invention, selection of the particular electrode configuration for delivery of the cardioversion or defibrillation pulses is controlled via output circuit 548, under control of cardioversion/defibrillation control circuitry 554 via control bus 546. Output circuit 548 switches the high voltage electrodes 506 and 508 for delivery of the defibrillation or cardioversion pulse regimen, and may also be used to specify a multi-electrode, simultaneous pulse regimen or a multi-electrode sequential pulse regimen. Monophasic or biphasic pulses may be generated. One example of circuitry which may be used to perform this function is set forth in U.S. Pat. No. 5,163,427, issued to Keimel, incorporated herein by reference. However, output control circuitry as disclosed in U.S. Pat. No. 4,953,551, issued to Mehra et al. or U.S. Pat. No. 4,800,883, issued to Winstrom both incorporated herein by reference, may also be used in the context of the present invention. Alternatively single monophasic pulse regimens employing only a single electrode pair according to any of the above cited references which disclose implantable cardioverters or defibrillators may also be used.

Operation of the System of the Present Invention

FIG. 3 is an illustration of detection interval ranges which may be employed in a preferred embodiment of the present invention. The specific detection interval ranges are selected and programmed by the physician. As seen, events which occur less than 120 milliseconds (hereafter "ms") apart are not detected due to blanking. This is a fixed interval and its length is not programmable by the physician. The range of intervals between detected events taken as indicative of fibrillation are greater than 120 ms and less than 300 ms. That is the fibrillation detection interval (hereafter "FDI") extends to 300 ms. This range is programmed and is selected by the physician to suit the particular patient. The range of intervals between detected events taken as indicative of tachyarrhythmia are greater than 300 ms and less than 450 ms. That is the tachyarrhythmia detection interval (hereafter "TDI") extends to 450 ms. This range is also programmed and is selected by the physician to suit the particular patient. Events having intervals between 450 ms to 923 ms, in the preferred embodiment, are taken as indicative of normal sinus rhythm. That is the brady escape interval (hereafter "BEI") extends to 923 ms. This range is also programmed and is selected by the physician to suit the particular patient. Events which occur at intervals which would be greater than the BEI are taken as indicative of bradycardia.

For example, if a first event is sensed and a second event is sensed 200 ms later, ventricular fibrillation is provisionally detected. As a second example, if a first event is sensed and second event occurs 100 ms later and a third event occurs 210 ms after the second event, then a ventricular tachycardia (hereafter "VT") is provisionally detected. This is so because the second event occurred during blanking and thus was not sensed; the third event was thereafter sensed a sum of 320 ms after the first, well within the VT zone.

It should be noted that the specific times for intervals is for the preferred embodiment and thus is only illustrative of the present invention. Other interval lengths may also be used within the scope of the present invention.

FIG. 4 is an arrhythmia detection/therapy muscle state diagram of the present invention. As discussed above the present invention features skeletal muscle graft stimulation as well as cardiac stimulation. One of the important requirements of such a system, however, is to accurately detect cardiac arrhythmias and respond with the appropriate therapy. As discussed above, concurrent skeletal muscle graft stimulation may interfere with the detection and diagnosis of arrhythmias. Thus, one important feature of the present invention is the manner in which it provides for skeletal muscle graft stimulation as well as cardiac stimulation while also managing the prompt detection and diagnosis of arrhythmias. In particular, the present invention temporarily stops or inhibits skeletal muscle stimulation once the onset of an arrhythmia is sensed.

As seen, during normal sinus rhythm the system remains at normal sinus rhythm state 30. In state 30 device provides both skeletal muscle graft stimulation and any bradycardia stimulation required. Bradycardia stimulation may take the form of any suitable electrical stimulation therapy, and preferably is given in the form of VVI pacing, although other types of pacing therapy may be delivered, such as VOO, OVO and VVT. Bradycardia stimulation is delivered, in the preferred embodiment, upon the detection of a sequence of cardiac events in which the range of intervals between detected events greater than BEI.

If, however, a sequence of cardiac events is detected in which the range of intervals between detected events is less than the TDI, then the skeletal muscle stimulation is inhibited (as represented by line 31) and VT counting mode state 32 is reached. In the preferred embodiment, if only one TDI is detected, then the skeletal muscle stimulation is inhibited and VT counting mode state 32 is reached.

While in the VT counting mode state 32, the skeletal muscle stimulation is re-enabled and the device returns to normal sinus rhythm state 30 if one interval greater than the TDI is detected.

In addition, when a sequence of cardiac events is detected in which the range of intervals between detected events is less than the FDI, then the skeletal muscle stimulation is inhibited (as represented by line 31) and VF counting mode state 34 is reached. In the preferred embodiment, if only one FDI is detected, then the skeletal muscle stimulation is inhibited and VF counting mode state 34 is reached.

While in the VF counting mode state 34, if VT detection is programmed on, the skeletal muscle stimulation is re-enabled and the device returns to normal sinus rhythm state 30 upon the detection of consecutive events with intervals greater than TDI equal to one-third of the number of intervals to detect VF (hereafter "VFNID"). If, however, VT detection is programmed off, the skeletal muscle stimulation is re-enabled and the device returns to normal sinus rhythm state 30 upon the detection of consecutive intervals greater than FDI equal to one-third of VFNID. Of course, if VT detection is programmed off, deliver VT therapy state 36 may still be reached through combined count state 38, discussed below.

It should be noted because FDI is smaller than TDI, then when VF counting mode state 34 is reached, this necessarily implies VT counting mode state 32 is also reached. From an electronic circuit design perspective, however, the counting bins for each state are simultaneously active, although both not necessarily registering events at the exact same time.

While in VT counting mode state 32 the device counts the number of events which meets the TDI criterion. When the cumulative VT event counter is equal to the number of intervals to detect VT, also called VTNID, then VT detection is fulfilled, deliver VT therapy state 36 is reached and VT therapy is delivered. In the preferred embodiment VTNID is programmable. As discussed in more detail below, VT detection and deliver VT therapy state 36 may also be reached through combined count state 38.

While in the VF counting mode state 34 the device counts the number of events which meet the FDI criterion. When the cumulative event counter is equal to VFNID, then VF detection is fulfilled, deliver VF therapy state 40 is reached and VF therapy is delivered. In the preferred embodiment VFNID is programmable. As discussed above, VFNID essentially is the number of past events that must satisfy the FDI criteria to be detected as fibrillation. The count uses past events that have been stored in the fibrillation event buffer memory (located in the RAM 526 of FIG. 2) which include both paced and sensed events. For example, if VFNID is set to 18 and fibrillation event buffer is set to 24; then to detect VF 18 of the last 24 events must satisfy the FDI criteria. As seen, deliver VF therapy state 40 may also be reached combined count state 38.

Combined count state 38 is provided to avoid excessive detection times during competing VT and VF counters. Thus combined count state 38 is reached, in the preferred embodiment, when the VF event counter reaches five and the VT event counter plus the VF event counter is greater than or equal to the combined number of intervals to detect parameter (hereafter "CNID"). In the preferred embodiment CNID is not directly programmable, but rather is equal to seven sixths of VFNID. Once the combined count state 38 is reached, then the second look criterion is applied.

Second look criterion is used only after combined count state 38 is reached. Second look criterion is applied to determine whether VT or VF therapy should be delivered. In the preferred embodiment second look criterion is as follows: If all of the previous 8 intervals are greater than or equal to FDI, then the VT detected path should be followed and deliver VT therapy state 36 is reached, but if one of the previous 8 intervals is less than FDI, then the VF detected path will be followed and deliver VF therapy state 40 is reached.

Once deliver VF therapy state 40 is reached, VF therapy is completed or aborted and VT/VF termination detection state 42 is reached. Similarly once deliver VT therapy state 36 is reached, VT therapy is completed or aborted and VT/VF termination detection state 42 is reached.

While in VT/VF termination detection state 42, the device determines whether VT or VF is re-detected. If either VT or VF is detected, then the device returns to the relevant therapy state. If neither VT nor VF is re-detected, the device returns to normal sinus state 30. VT/VF termination detection is accomplished as follows: If VT detection is programmed "Off" and eight consecutive events having intervals greater than FDI are sensed, then VF termination is detected and the device returns to normal sinus state 30. If VT detection is programmed "On" and eight consecutive events having intervals greater than TDI (which by definition is greater than FDI) are sensed, then VT termination is detected and the device returns to normal sinus state 30.

As discussed above the present invention also features skeletal muscle stimulation while charging for defibrillation. Essentially this feature provides muscle stimulation pulses to the grafted skeletal muscle while the device is charging a capacitor to deliver a defibrillation pulse. As mentioned above, because the muscle continues to contract and causes cardiac perfusion to be maintained. This cardiac perfusion, in turn, limits the increase in the overall defibrillation threshold. Because the increase in these thresholds is minimized, this permits the device to feature smaller capacitors or lower voltages or both.

FIG. 5 is a timing diagram showing the relationship between muscle stimulation, cardiac events and a defibrillation charge cycle. As seen, during normal sinus rhythm, represented here by normal QRS complex 202 the device is in normal sinus state 30. As such, muscle stimulation burst 201 is delivered to stimulate the skeletal muscle graft and thereby provide cardiac assistance, as described above. At first occurrence of a VF event 204 device enters detection state 206. As explained in FIG. 4, during detection state 206 device is in VF counting mode state 34 and VT counting mode state 32. As also explained in FIG. 4 once a VF event 204 is detected all muscle stimulation is inhibited, as may be seen in the lack of any muscle bursts in the region of detection state 206. Once VF is confirmed the device then enters deliver VF therapy state 40.

While in deliver VF therapy state 40, device performs several operations, including charging of the output capacitors, depicted as line 208. In addition, skeletal muscle stimulation is re-initiated and a series of asynchronous muscle stimulation bursts 210, 212 are delivered. In the preferred embodiment asynchronous bursts 210, 212 have a greater amplitude than muscle stimulation burst 201, on the order of one and a half times as large.

Once charging of the output capacitors is completed, a sequence to synchronize the defibrillation discharge to a sensed R-wave is undertaken. In particular, device begins a synchronization sequence during synchronization time 216. Synchronization sequence is undertaken to synchronize defibrillation discharge to a sensed cardiac event as well as to re-confirm the presence of the arrhythmia. If the synchronization sequence is successful, then defibrillation discharge 214 is delivered synchronized to a sensed cardiac event. If the synchronization sequence is unsuccessful, then defibrillation discharge 214 is delivered at the timing out of synchronization time 216. In addition during synchronization time 216, device re-inhibits skeletal muscle stimulation in order to permit reliable sensing of any intrinsic cardiac events.

FIG. 6 is a timing diagram showing the relationship between muscle stimulation and cardiac events of an alternate embodiment. In particular, in an alternate embodiment, if synchronization is unsuccessful, then the device delivers an asynchronous muscle stimulation burst 322 immediately prior to defibrillation discharge 214, as best seen in FIG. 6. Muscle stimulation burst 322 is intended to cause the heart to be squeezed by the skeletal muscle graft and achieve roughly a systolic position when defibrillation discharge 214 is delivered. Because the volume of the heart in such a position is decreased the defibrillation threshold is likewise decreased.

Turning again to FIG. 5, once defibrillation discharge 214 is delivered, then device enters into VT/VF termination detection state 42 to thereby confirm heart has returned to normal sinus rhythm.

FIG. 7 depicts an alternate muscle stimulation burst which may be used with the present system. These muscle stimulation bursts may be used at any suitable time within the present system, and are not limited to only use prior to delivery of the defibrillation therapy. As seen muscle stimulation burst 300 occurs after QRS 303 in the amount of a synchronization delay 305. In the preferred embodiment synchronization delay 305 is programmable and is undertaken in order to synchronize the muscle stimulation burst 300 with the ventricular contraction. Muscle stimulation burst 300 has essentially two sections, first section 301 and second section 302, often referred to as "muscle catch" and "muscle pulse train" respectively. As seen, first section 301 has a smaller interpulse interval 304 within the burst, i.e. a higher frequency. In comparison second section 302 has a relatively larger interpulse interval 304 within the burst, i.e. a relatively smaller frequency. The higher frequency first section 301 increases the velocity and force of the skeletal muscle graft contraction. In the preferred embodiment interpulse interval 304 and number of pulses in the catch may be selected by the physician. The pulse waveform, amplitude 308 and width of the muscle catch are the same for the remainder of the burst.

FIG. 8 depicts an alternate embodiment of the muscle catch stimulation which may be used with the present system. As seen all parameters of the muscle stimulation burst 300 are the same as that described above with respect to FIG. 7 but for the amplitude of second section 302.

FIG. 9 depicts an alternate embodiment of the muscle catch stimulation which may be used with the present system. As seen all parameters of the muscle stimulation burst 300 are the same as that described above with respect to FIG. 7 but for the amplitude of second section 302. In particular amplitude of each burst within second section 302 decreases. The rate of decrease of pulse amplitude within each burst decreases as a function of rate, i.e. the faster the rate of muscle stimulation, the greater the decrease of pulse amplitude within the pulse train.

FIG. 10 illustrates a potential problem which could occur in a device which delivers simultaneous muscle burst stimulation while also charging a defibrillation output capacitor. As seen, during VF 204 device enters VF detection state 206. As explained in FIG. 4, during VF detection state 206 device is in VF counting mode state 34 and VT counting mode state 32. As also explained in FIG. 4 once a VF event 204 is detected all muscle stimulation is inhibited, as may be seen in the lack of any muscle bursts in the region of VF detection state 206. Once VF is confirmed the device then enters deliver VF therapy state 40.

While in deliver VF therapy state 40, the device performs several operations, including charging of the defibrillation output capacitors as well as re-initiating muscle stimulation burst. During deliver VF therapy state 40 device initiates charging of the output capacitors, depicted as line 208. In addition, device also re-initiates (recall muscle burst stimulation is turned-off during VF detection state 206 to facilitate sensing) and a series of asynchronous muscle stimulation bursts 210, 212, 228 are delivered. Muscle stimulation bursts 210, 212, 228, however, increase the drain on battery as well as retarding the output capacitor from reaching the desired voltage. In particular output capacitor voltage is depicted by line 208. Upon examination it should be noted that the rate of increase in the output capacitor voltage depicted as line 208 is less in the regions 221, 223 and 225, which correspond to muscle stimulation bursts 210, 212 and 228 respectively. Besides increasing the time required to charge output capacitor, the concurrent provision of muscle burst stimulation along with the charging of the output capacitor may also cause the battery voltage to temporarily fall to an level where proper device functionality is not guaranteed. As seen battery voltage 220 inversely corresponds to output capacitor voltage 208. In particular battery voltage 220 more rapidly falls in the regions 222, 224 and 226, which correspond to muscle stimulation bursts 210, 212 and 228 respectively. As also seen, battery voltage 220 almost, in the example illustrated, falls to power-on-reset level 228. It is extremely important that battery voltage 208 not fall to this level. As explained above, upon reaching the POR threshold the device would reset itself. Functionality would be temporarily lost and all ongoing therapies interrupted or aborted. The result would be any defibrillation therapy would not be delivered until a significant amount of time had elapsed.

A delay in the amount of the synchronization time 216 before defibrillation discharge 214 is delivered is designed to confirm the continued presence of the arrythmia as well as an attempt to also synchronize the defibrillation discharge 214 to an R-wave.

Operation of the Battery Voltage Safety Limit

FIG. 11 illustrates the operation of the battery voltage safety limit to inhibit the deliver of muscle stimulation pulses while the defibrillation output capacitor is being charged. As seen, during VF 204 device enters detection state 206. As explained in FIG. 4, during detection state 206 device is in VF counting mode state 34 and VT counting mode state 32. As also explained in FIG. 4 once a VF event 204 is detected all muscle stimulation is inhibited, as may be seen in the lack of any muscle bursts in the region of detection state 206.

Once VF is confirmed the device then enters deliver VF therapy state 40. While in this state, device performs several operations, including charging of the defibrillation output capacitors as well as re-initiating muscle stimulation burst. During deliver VF therapy state 40 device initiates charging of the output capacitors, depicted as line 208. In addition, device also re-initiates (recall muscle burst stimulation is turned-off during VF detection state 206 to facilitate sensing) and a series of asynchronous muscle stimulation bursts 210, 212 are delivered. Muscle stimulation bursts 210 and 212, however, increase the drain on battery as well as retard the output capacitor from reaching the desired voltage. Upon examination it should be noted that the rate of increase in the output capacitor voltage 208 is less in the regions 221 and 223, which correspond to muscle stimulation bursts 210 and 212 respectively. Besides increasing the time required to charge output capacitor, the concurrent provision of muscle burst stimulation along with the charging of the output capacitor may also cause the battery voltage to temporarily fall to an unacceptable level. As seen battery voltage 220 inversely corresponds to output capacitor voltage 208. In particular battery voltage 220 more rapidly falls in the regions 222 and 224, which correspond to muscle stimulation bursts 210 and 212 respectively. As also seen, battery voltage 220 falls to battery voltage safety limit 229. Once reached, this cause the device to inhibit further delivery of muscle stimulation bursts, as illustrated by the doted lines 230. It should be also noted the suspension of skeletal muscle bursts causes the output capacitor voltage 208 to increase more rapidly in the region 224. Ultimately, besides insuring that proper functionality of the device is maintained at all times, this also hastens the delivery of defibrillation pulse 214.

FIG. 12 is a flowchart showing the operation of the system and in particular the operation of the battery voltage safety limit feature of the present invention. As seen, at 206 system confirms the presence of an arrythmia. If an arrythmia is not confirmed and instead normal sinus rhythm is confirmed at 250, then the system returns to normal sinus rhythm state 30, as discussed in FIG. 4. If an arrythmia is confirmed, then the system goes to 251 and determines whether the high voltage defibrillation capacitors are charged. If the high voltage defibrillation capacitors are charged, then the system begins synchronization of the delivery of the therapy at 216. As discussed above, at 216 device may deliver a high amplitude muscle stimulation burst 322 as discussed in FIG. 6.

If the high voltage defibrillation capacitors are not fully charged, then the system continues to charge at 252. Thereafter, the system proceeds to 253 and determines whether battery voltage 220 is greater than battery voltage safety limit 229. If greater than, then the system proceeds to 254 deliver any muscle stimulation pulses within a burst which may be scheduled for delivery at that time. Once any such pulse is delivered, then the system proceeds again to 251 to determine whether the high voltage defibrillation capacitors are charged. As seen, at this point the cycle repeats itself. If the system determines at 253, however, the battery voltage 220 is not greater than the battery voltage safety limit 229, then the system proceeds to 255 and stops delivery of the muscle stimulation burst. The system than proceeds again to 251 to determine whether the high voltage defibrillation capacitors are charged. As seen at this point the cycle repeats itself until such time as the high voltage capacitors are charged so the system could begin synchronization of the delivery of the therapy at 216.

FIG. 13 is a flowchart showing the operation of an alternate embodiment of the battery voltage safety limit feature of the present invention. As seen, at 206 system confirms the presence of an arrythmia. If an arrythmia is not confirmed and instead normal sinus rhythm is confirmed at 250, then the system returns to normal sinus rhythm state 30, as discussed in FIG. 4. If an arrythmia is confirmed, then the system goes to 251 and determines whether the high voltage defibrillation capacitors are charged. If the high voltage defibrillation capacitors are charged, then the system begins synchronization of the delivery of the therapy at 216. As discussed above, at 216 device may deliver a high amplitude muscle stimulation burst 322 as discussed in FIG. 6.

If the high voltage defibrillation capacitors are not fully charged, then the system continues to charge at 252. Thereafter, the system proceeds to 253 and determines whether battery voltage 220 is greater than battery voltage safety limit 229. If greater than, then the system proceeds to 254 deliver any muscle stimulation pulses within a burst which may be scheduled for delivery at that time. Once any such pulse is delivered, then the system proceeds again to 251 to determine whether the high voltage defibrillation capacitors are charged. As seen, at this point the cycle repeats itself. If the system determines at 253, however, the battery voltage 220 is not greater than the battery voltage safety limit 229, then the system proceeds to 255 and stops delivery of the muscle stimulation burst. The system than proceed to 256 to continue charging the high voltage defibrillation capacitors. Once the high voltage defibrillation capacitors are charged, then the system begins synchronization of the delivery of the therapy at 216. Thus the alternate embodiment functions to inhibit all delivery of muscle stimulation pulses, once the battery voltage safety limit has been reached, until after the high voltage capacitors have been charged and the therapy delivered. In such a manner the alternate embodiment prioritizes the deliver of the defibrillation therapy over the delivery of muscle stimulation therapy once a charge cycle is begun.

As discussed above, the present invention provides a system which shuts down low current circuitry in the system in the event that battery voltage drops to a level such that the regulated output voltage is endangered, while maintaining the charging of the high voltage, high current circuitry. In particular the present invention prevents the dual demands of skeletal muscle stimulation along with charging of the defibrillation output capacitor from causing the battery voltage to collapse and drop below the power-on-reset threshold.

While the present invention has been described in detail with particular reference to a preferred embodiment, it will be understood variations and modifications can be effected within the scope of the following claims. Such modifications may include substituting elements or components which perform substantially the same function in substantially the same way to achieve substantially the same result for those described herein. 

What is claimed is:
 1. A device for delivering electrical stimulation to a first tissue and a second tissue comprising:a battery, the battery having a battery voltage; means for delivering high voltage electrical stimulation to a first tissue, the means for delivering high voltage electrical stimulation coupled to the battery; means for delivering low voltage electrical stimulation to a second tissue, the means for delivering low voltage electrical stimulation coupled to the battery; means for detecting the battery voltage; and means for temporarily inhibiting the means for delivering low voltage electrical stimulation when the detected battery voltage reaches a first level.
 2. The device of claim 1 wherein the means for delivering high voltage electrical stimulation comprises a capacitor.
 3. The device of claim 1 wherein the means for delivering high voltage electrical stimulation to a first tissue comprises means for delivering defibrillation pulses to cardiac tissue.
 4. The device of claim 1 wherein the means for delivering low voltage electrical stimulation to a second tissue comprises an intramuscular lead.
 5. The device of claim 1 wherein the means for delivering low voltage electrical stimulation to a second tissue comprises means for delivering muscle stimulation bursts, each burst having at least two pulses.
 6. An apparatus for stimulating a skeletal muscle, detecting cardiac events and delivering therapeutic stimulation to the heart comprising:a battery, the battery having a battery voltage; means for delivering high voltage defibrillation stimulation to a heart, the means for delivering high voltage defibrillation stimulation coupled to the battery; means for delivering low voltage electrical stimulation to skeletal muscle, the means for delivering low voltage electrical stimulation coupled to the battery; means for detecting the battery voltage; and means for inhibiting the means for delivering low voltage electrical stimulation when the detected battery voltage reaches a first level.
 7. The apparatus of claim 6 further comprising means for re-initiating the means for delivering low voltage electrical stimulation when the detected battery voltage reaches a second level.
 8. The apparatus of claim 6 wherein the means for delivering high voltage electrical stimulation to the heart comprise means for delivering defibrillation pulses to the heart.
 9. An apparatus for stimulating skeletal muscle and delivering high voltage electrical energy to a heart comprising:a battery, the battery having a battery voltage; means for delivering stimulation to the skeletal muscle, the means for delivering stimulation to the skeletal muscle coupled to the battery; means for temporarily storing high voltage electrical energy; means for charging the means for temporarily storing energy, the means for charging coupled to the battery, the means for charging coupled to the means for temporarily storing high voltage electrical energy; means for detecting the battery voltage; and means for inhibiting the means for delivering stimulation to a skeletal muscle grafted about a heart upon the detection of a first battery voltage.
 10. The apparatus of claim 9 further comprising means for re-initiating the means for delivering stimulation to the skeletal muscle upon the detection of a second battery voltage.
 11. A device for delivering electrical stimulation to a first tissue and a second tissue comprising:a battery, the battery having a battery voltage; means for delivering high voltage electrical stimulation to a first tissue, the means for delivering high voltage electrical stimulation coupled to the battery; means for delivering low voltage electrical stimulation to a second tissue, the means for delivering low voltage electrical stimulation coupled to the battery; means for detecting the battery voltage; and means for inhibiting the means for delivering low voltage electrical stimulation when the detected battery voltage reaches a first level until after the means for delivering high voltage electrical stimulation have been charged by the battery. 